Lecture 3b— Introduction to semiconductors

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Text of Lecture 3b— Introduction to semiconductors

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Lecture 3

    Introduction to Semiconductors and Energy Bandgaps

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Solar Cells

    Why do the electrons flow when light is present but not flow when light is not present?

    Answer, Energy Bandgap (very important concept).

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    For metals, the electrons can jump from the valence orbits (outermost core energy levels of the atom) to any position within the crystal (free to move throughout the crystal) with no extra energy needed to be suppliedFor insulators, it is VERY DIFFICULT for the electrons to jump from the valence orbits and requires a huge amount of energy to free the electron from the atomic core.For semiconductors, the electrons can jump from the valence orbits but does require a small amount of energy to free the electron from the atomic core.

    Classifications of Electronic Materials

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Semiconductor materials are a sub-class of materials distinguished by the existence of a range of disallowed energies between the energies of the valence electrons (outermost core electrons) and the energies of electrons free to move throughout the material.The energy difference (energy gap or bandgap) between the states in which the electron is bound to the atom and when it is free to conduct throughout the crystal is related to the bonding strength of the material, its density, the degree of ionicity of the bond, and the chemistry related to the valence of bonding.High bond strength materials (diamond, SiC, AlN, GaN etc...) tend to have large energy bandgaps.Lower bond strength materials (Si, Ge, etc...) tend to have smaller energy bandgaps.

    Classifications of Electronic Materials

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Classifications of Electronic Materials

    More formally, the energy gap is derived from the Pauli exclusion principle, where no two electrons occupying the same space, can have the same energy. Thus, as atoms are brought closer towards one another and begin to bond together, their energy levels must split into bands of discrete levels so closely spaced in energy, they can be considered a continuum of allowed energy. Strongly bonded materials tend to have small interatomic distances between atoms. Thus, the strongly bonded materials can have larger energy bandgaps than do weakly bonded materials.

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Material Classifications based on Bonding MethodBonds can be classified as metallic, Ionic, Covalent, and van der Waals.

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Consider the case of the group 4 elements, all** covalently bonded

    Element Atomic Radius/Lattice Constant Bandgap(How closely spaced are the atoms?)

    C 0.91/3.56 Angstroms 5.47 eV

    Si 1.46/5.43 Angstroms 1.12 eV

    Ge 1.52/5.65 Angstroms 0.66 eV

    -Sn 1.72/6.49 Angstroms ~0.08 eV*

    Pb 1.81/** Angstroms Metal

    *Only has a measurable bandgap near 0K

    **Different bonding/Crystal Structure due to unfilled higher orbital states

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Classifications of Electronic MaterialsTypes of Semiconductors:Elemental: Silicon or Germanium (Si or Ge)Compound: Gallium Arsenide (GaAs), Indium Phosphide (InP), Silicon Carbide (SiC), CdS and many others

    Note that the sum of the valence adds to 8, a complete outer shell. I.E. 4+4, 3+5, 2+6, etc...

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Compound Semiconductors: Offer high performance (optical characteristics, higher frequency, higher power) than elemental semiconductors and greater device design flexibility due to mixing of materials.

    Binary: GaAs, SiC, etc...

    Ternary: AlxGa1-xAs, InxGa1-xN where 0

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Material Classifications based on Crystal Structure

    Amorphous MaterialsNo discernible long range atomic order (no detectable crystal structure). Examples are silicon

    dioxide (SiO2), amorphous-Si, silicon nitride (Si3N4), and others. Though usually thought of as less perfect than crystalline materials, this class of materials is extremely useful.Polycrystalline Materials

    Material consisting of several domains of crystalline material. Each domain can be oriented differently than other domains. However, within a single domain, the material is crystalline. The size of the domains may range from cubic nanometers to several cubic centimeters. Many semiconductors are polycrystalline as are most metals.

    Crystalline MaterialsCrystalline materials are characterized by an atomic symmetry that repeats spatially. The shape of

    the unit cell depends on the bonding of the material. The most common unit cell structures are diamond, zincblende (a derivative of the diamond structure), hexagonal, and rock salt (simple cubic).

    Classifications of Electronic Materials

    Increasing structural quality and increasing efficiency (performance)

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Chemical Bonding Determines the

    Bandgap

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    Comparison of the Hydrogen Atom and Silicon Atom

    Hydrogen

    Silicon

    ...3,2,1,arg2/2/tan,

    6.13)4(2 220

    4

    nandechelectronqhtconsplanksmasselectronmwhere

    neV

    nqmEnergy

    o

    oelectronHydrogen

    n=1: Complete Shell2 s electrons

    n=2: Complete Shell2 2s electrons6 2p electrons

    n=3:2 3s electronsOnly 2 of 6 3p electrons

    4 empty states

    4 Valence Shell Electrons

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Pauli Exclusion Principle

    Only 2 electrons, of spin+/-1/2, can occupy the same energy state at the same point in space.

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    Banding of Discrete states and the Simplified Model

    T=0K

    EC or conduction band

    EV or valence band

    Band Gap where no states

    exist

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    4 electrons available for sharing (covalent bonding) in outer shell of atoms

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Ec

    Ev

    No Holes valence band means no hole conduction is possible

    No electrons in conduction band means no electron conduction

    is possible

    Band Occupation at Low Temperature (0 Kelvin)

    For (Ethermal=kT)=0

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    +

    Ec

    Ev

    Electron free to move in conduction band

    Hole free to move in valence band

    Band Occupation at Higher Temperature (T>0 Kelvin)

    For (Ethermal=kT)>0

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    +

    For (Ethermal=kT)>0

    Carrier Movement Under Bias

    Direction of Current Flow

    Direction of Current Flow

    Ec

    Ev

    Electron free to move in conduction band

    Hole movement in valence band

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    +

    Ec

    Ev

    Electron free to move in conduction band

    Hole movement in valence band

    Carrier Movement Under Bias

    Direction of Current Flow

    Direction of Current Flow

    For (Ethermal=kT)>0

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    +

    Ec

    Ev

    Carrier Movement Under Bias

    Electron free to move in conduction band

    Hole movement in valence band

    Direction of Current Flow

    Direction of Current Flow

    For (Ethermal=kT)>0

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    +

    Ec

    Ev

    Carrier Movement Under Bias

    Electron free to move in conduction band

    Hole movement in valence band

    Direction of Current Flow

    Direction of Current Flow

    For (Ethermal=kT)>0 QuickTime Movie

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Ec

    Ev

    The valance band may have ~4e22 cm-3 valence electrons participating in the bonding processes holding the crystal together.

    The valance band might only have ~1e6 to 1e19 cm-3 holes in the valence band (missing valence electrons). Thus, it is easier to account for the influence of the holes by counting the holes directly as apposed to counting very small changes in the valence electron concentrations.

    Example: If there are 1e 22 cm-3 atoms in a crystal with each atom having 4 valence electrons. What is the difference in valence electron concentration for 1e12 holes verses 1e13 cm-3 holes?

    Answer: 4 x 1e22 cm-3 -1e12 cm-3 = 3.9999999999e22cm-3 verses

    4 x 1e22 cm-3 -1e13 cm-3 = 3.999999999e22cm-3

    For accounting reasons keeping track of holes is easier!

    Clarification of confusing issues:Holes and Electrons

  • ECE 4833 - Dr. Alan DoolittleGeorgia Tech

    Clarification of confusing issues:Holes and ElectronsTerminology

    Electrons: Sometimes referred to as conduction electrons: The electrons in the conduction band that are free to move throughout the crystal.

    Holes: Missing electrons normally found in the valence band (or empty states in the valence band that would normally be filled).

    If we talk about empty states in the conduction band, we DO NOT call them holes! This would be confusing. The conduction band h